Nonmagnetic iron-base alloys



March 12, 1963 J.H.SCHRAM1M 3,081,164

NONMAGNETIC IRON-BASE ALLOYS Filed Nov. 4, 1959 WITNESSES INVENTOR 2% vJoggb H. Schromm WA ATTiNEY United States Patent Ofiice 3,081,164Patented Mar. 12, 1963 3,081,164 NONMAGNETIC IRON-BASE ALLOYS Jacob H.Schranim, Bayonne, Ni, assignor to Westinghouse Electric Corporation,East iittsburgh, Pin, a corporation of Pennsylvania Fiied Nov. 4, 1959,'Ser. No. 850,822 2 Claims. (Cl. 75-123) This invention relates toimproved nonmagnetic ironbase alloys, and members prepared therefrom.

Certain austenitic alloys of iron are particularly suitable forapplications demanding strong, nonmagnetic material. In addition, thesealloys are valued for the excellent resistance to corrosion, and for thehigh temperature properties, which may be imparted to them byappropriate additives. Nickel is ordinarily specified for inclusion inthese alloys to establish the austenitic structure. While nickel isexcellent for this purpose, because of its strategic need and relativelyhigh cost, the replacementof nickel with a readily availableand lessexpensive additive is highly desirable.

One group of alloys of the austenitic type is the stainless steels, themost common being 18% chromium, 8% nickel, balance iron with a smallamount of carbon (from .08% to 20%). The ordinary austenitic stainlesssteels are not susceptible to precipitation hardening by heat treatmentand this is disadvantageous where high hardness and strength isrequired. In order to provide alloys which are hardenable by heattreatment, alloys have been made 'wherein the carbon content is higherthan the carbon content in stainless steel, and wherein the chromiumcontent is lower than that in stainless steel (resistance to corrosionnot being critical in the proposed application). The additional hardnesswhich is obtainable in such alloys is secured by the precipitation ofcarbides during heat treatment. The particular carbide which isprecipitated is probably chromium carbide, although other carbides mayalso be involved.

In the construction of large inner-cooled tu-rbogenerators, theretaining ringsand wedges used are preferably manufactured from a highstrength, substantially nonmagnetic material to resist the strainsplaced upon the members, and to avoid paths of magnetic leakage whichwould decrease the efiiciency of the generators. The function of theretaining rings and wedges is tohold in position on the generator rotorthe heavy conductors which carry the excitation field current. At thenormal speed of a two pole generator, 3600 r.p.m., an intensecentrifugal force is developed which tends to throw the conductors oh?the rotor, and the retaining rings and wedges that secure the conductorsin place against this great force must necessarily be formed from amaterial having high strength. In general, a 02% yield strength of100,000 pounds per square inch (p.s.i.) is a minimum requirement forthis application. This strength level is not particularly .diflicult toreach in low alloy magnetic steels, but the use of such magneticmaterials is undesirable because they limit the efficiency of thegenerators to whichthey are applied. The eddy currents induced inmagnetic retaining rings due to their proximity to conductors carrying'heavy'currents, represent wasted energy which latter, on continuedoperation, takes the form of heat generated in the rings. Thus, not onlyis there a .loss of energy, but in heating, the rings reach excessivetemperature and the operating conditions of the generator are adverselyaffected. With nonmagnetic rings this energy loss'is negligible and therings will remain relatively cool.

The steels which are presently employed for generator retainer rings andwedges are of the type to which reference has been made above; that is,they are austenitic in structure. andirely at least in, part, on theprecipitation hardening phenomenon characterized by the precipitation ofcarbides to achieve desired strength and hardness levels. It has beenfound, in actual practice, that the ordinary precipitation hardeningprocedure, i.e., formation of a solid solution, and cooling to the agingtemperature and holding at this temperature to cause precipitation ofhardening components, is not suffioient to produce the requiredcombination of strength and ductility (elongation). It has been foundnecessary to cold work the alloys before or during aging to attain thedesired level of mechanical properties. In these alloys, work hardeningand precipitation hardening occur substantially simultaneously. Thus,the alloys harden so readily during the cold working that they becomeexcessively hard and the amount of reduction which can be eifected isseverely limited. Therefore, even though high capacity tools areemployed to perform the cold working, the high hardness of the alloy asit is being worked prevents thorough working of the alloy member,consequently, the amount of deformation varies widely over the crosssection of the worked member, and hence, the mechanical propertiessimilarly show wide variation and nonuniformity within the individualmember.

Accordingly, it is a principal object of this invention to provide areadily workable nonmagnetic austenitic iron-base alloy having highstrength and hardness, which is precipitation hardenable and employs aminimum amount of nickel.

It is another object of this invention to provide a precipitationhardenable nonmagnetic iron-base alloy having predetermined amounts ofmanganese and titanium therein.

It is still another object of this invention to provide precipitationhardenable, substantially nonmagnetic, ironbase alloys havingpredetermined amounts of nickel, manganese, and titanium therein.

It is a further object of this invention to provide generator retainingrings and wedges having high strength and hardness, from an alloyincluding a minimum of carbon and nickel, which may be discretely workhardened and precipitation hardened.

Other objects of this invention will, in part, be obvious, and will, inpart, appear hereinafter.

A clearer understanding of the function of the members formed from thealloys of this invention may be gained by reference to the drawings, inwhich:

FIGURE 1 is a fragmentary view in perspective of a generator rotorshowing elements made from the alloy of this invention, and

FIG. 2 is a fragmentary view in section taken along the line 11-11 ofFIG. 1; and

PEG. 3 is a fragmentary view in section taken along the line llllll ofFIG. 1.

This invention is directed to substantially non-magnetic iron-basealloys capable of both work hardening and precipitation hardening whichcontain a minimum amount of carbon and nickel. More particularly, thealloy comprises, byweight, from 9% 'to 30% manganese, from 0 to 9%nickel, the total of nickel and manganese lying in the range from 16% to30%, from 1.5% to 3.5%" titanium, from 0 to 12% chromium, a maximum of08% carbon, and the balance iron, with small amounts of additives, andincidental impurities. Other additives which may sometimes be present inrelatively small amounts are molybdenum, aluminum and silicon. Among theimpurities which may be present are phosphorus, sulfur, oxygen andnitrogen.

An alloy of the type described above which has been found to beparticularly good comprises, byweight, 25% to 26% manganese, from 1.8%to 3.5% titanium, a maxi mum of .03% carbon, and the balance essentiallyiron with the usual small amounts of impurities therein.

3 As the term nonmagnetic is employed in this descrip tion, it isintended to include alloys exhibiting no magnetic characteristics, andalso those loys which are only slightly magnetic. Some alloys of thisinvention, under which strengths would be desirable in many instances.This strength level is diificult to attain with consistency, and isparticularly diilicult to reach in alloy No. 1. It should be understoodthat while the average strength may Manufacturers of these alloys arevery reluctant to guarantee ultimate tensile strengths of over 125,000p.s.i.,

certain conditions, exhibit very slight magnetic properties. 5 be high,a substantial proportion of alloy members will Referring to FIGS. 1 and2 of the drawings, there is be considerably less than this average. 2shown a two pole rotor having a rotor body 11 with The compositions ofthe alloys of this invention are bearing shafts 12 at opposite ends ofthe rotor body. set forth, in weight percent, in Table III.

The rotor body 11 has smooth pole faces 13 at opposite Table III sidesof the body, only one of which is visible in FIG. 1. 10

Between the pole faces 13 the rotor body has a series of P h deeplongitudinal slots 14 which are machined parallel to Alloy M1 or N1 M0Ba] 0t m the axis of the rotor body. The slots as shown, extend 0 2 2substantially radially into the body, though they may also 321 III: 219

be parallel to each other. A cross-sectional view of the 15 slots 14 maybe seen in FIG. 2 where it will be observed 1 1 that in the opposingside walls of each of the slots 14 are 3-;

a pair of opposed longitudinal grooves 17. In each of 1110 6:8 33

the slots 14 is positioned a coil winding 15 with the outermost surfaceof the coil winding fiush with the innermost wall of the opposed grooves17. All the coil windings in the rotor taken together constitute thefield coil of the is; 1 g 21: 5: 2 ?i fig g g g gs g i 122 33 222 33%generator.

hs lmmedlately below. A plurahty of wedges 18 having T-shaped crosssections the paragrap (as seen in FIG. 2) are slidably mounted in eachof the 20 i "gg g fs g gfig i gg zi g ge g i' 523 32 slots 14. Thewedges 18 extend into the grooves 17 and 6 72 h um t and 24 at C arearranged in abutting relation with the coil windings A110 d 8 haattrez'lted M1190. C uenched and with each other, and thus both the coilwindings and 1d fled b 3 t C 3 ho S the wedges are restrained againstradial movement. gi g on g go C ag ur However, the ends of the coilwindings extend beyond a A110 v g s 1 g g treat d C chad the slots andrequire restraining means to hold them in i y M t 5 5 C d 93% 300 place.This is accomplished by the use of retaining rings. 3 C a u 0 $25 3 aretammg ring {elated coll Wmdmg Alloy P was solution heat treated at1050 C., quenched re 1s set fonth. In FIG. 3 it 1s seen that the end 01d11 d b t 507 t C d d 300 windings 21 of the coil windings 15, closelysurround the 1 2 3 a W a a an age Fowl. 11 whlch has F rfaduced endportion 11 adjacla'nt Alloy 6 was solution heat treated at 1050 Cquenched l- The wmdmgs are t have in oil cold rolled about 50% at 700 cand a ed 300 condu1ts 22 thereln for the c1rculat1on of hydrogen gashoursat c g or other. Smtable i A retammg. ring 23 is i a 40 Alloy 5 wassolution heat treated at 1050 C quenched close fitt1ng, surround1ngengagement w1th the end w1ndin oil cold rolled abzmt 50% at C d 3 ed 300ings preventmg rad1al or axial displacement of the windboursat C g ings.The retaining ring 23 may be secured to the rotor All o 12 was solut1onheat treated at 1120 C., at its po1nts of contact therewith in anyconvent1onal quenchxd in on, cold rolled about 50% at C and mammaged 300hours at 550 c.

The composmons of two prior art alloys which are Alloy 9 was Solutionheat treated at 11700 C presenfly used as genemtor rings and wedges areset quenched in oil, cold rolled about at 700 C. and forth Table I: T blI aged hours at 550 C.

a 9 Alloy 1267 was solution heat treated at 1140 C., [Composition wt.percent] 50 quenched in oil, cold rolled about 50% at 700 C., and

aged 120 hours at 550 C. Alloy N1 Mn Cr Mo 0 Al st Ti Fe Alloy U wassolution heat treated at 1160 C., quenched in oil, cold rolled about 50%at 700 C., and

g 9 g 2 0.4 5 aged 120 hours at 550 C.

"""""""""""""" 55 T "5 Ime hanical properties developed by the alloys ofThe mechamoal propertles character1st1c of these alloys, Table 6 Vor andheat trcatm nt are hsted m together with present and proposed commercialn1echanical property standards for alloys of this type are set forth ale IV for purposes of comparison in Table II. 6

O 0.02 Per- 02? 'r Table II Alloy scanty gang lsltltimattltla ga ing i ir l iii fi tff 1 I! [lvteehanicalpropernesl sgeiiggh sgesng gh (52 .5)iglil 00$? (P i c e nt) .l. A I A g 0.2 i r izi ht Ul ii rii at eElongation Reduction 111 300 00 Alloy Yield Tensile in 2" of Area1221000 133350 288 Sgageggggh Sg cggg er t) (Percent) 103,580 11510001401000 2331800 2326 44:1

112' an la l iii 1 25,5 0 20.4 00.4 1 120x10 135x10 25 33 120-139x1015e-105 10 26-31 42-40 Standard: 1303550 Present 115x10 125x10 15 20 IProposed. 120x10 x10 20 40 The schedules for working and heat treatmentlisted above will yield satisfactory products, but are by no means theonly acceptable procedures. For example, the

5. precipitation times may be reduced by increasing the agingtemperature somewhat. Increasing the aging temperature, however, doeshave the disadvantage that the ductility will usually be decreased. Theprecipitation times can also be shortened by increasing the amount ofcold work. 7

It will be noted that alloys Q and O which do not exhibit magneticproperties and contain neither nickel nor chromium, more than satisfyboth the present and proposed commercial standards for alloys for thisapplication. Seven other alloys, which contain moderate amounts ofchromium and nickel, also satisfy the en gineering requirements for theproposed application. Al oy V, however, containing very little nickeland over 20% manganese, falls just short of the desired strength level.The strength of thisalloy may be raised to the desired level by loweringthe aging temperature or increasing the aging time. The character ofthese alloys is such that, with proper heat treating and working,uniform and consistent mechanical properties are readily obtained. Withthese alloys, as with the alloys of Table I, the required combination ofstrength and ductility cannot be reached if, after hot working such asforging, the conventional precipitation hardening procedure of solutionheat treatment, rapid cooling, and aging is used. Cold working to effecta reduction of at least 30% before or during aging must be applied toobtain the required combination of mechanical properties.

In alloys Q and O, manganese acts alone to establish the austeniticstructure. The alloys other than Q and all have substituted therein fora part of the manganese, varying amounts of nickel, up to about 9%maximum, to supplement the action of manganese in promoting theformation of austenite. In these latter alloys it is the total ofmanganese and nickel which imparts the austenitic structure. The totalof manganese and nickel may range between 16% to 30%, but the preferredrange is to In this preferred range, manganese is present in amountsfrom 11% to 25%, by weight. The high proportion of manganese in thealloys of this invention serves also to improve impact strength at lowtemperatures.

The titanium in the alloy, which lies in the range 1.5% to 3.5%, andpreferably in the range 2.25% to 3.25%, is present to promoteprecipitation hardening by the formation of compounds with iron andperhaps other elements in the alloy.

Although chromium is not essential to the alloys of this invention, upto 12% chromium may be added where it is desired to improve thecorrosion resistance of the alloys.

In the alloys of this invention carbon is purposely maintained at a lowlevel in order to avoid the shortcomings of the prior art alloys. Forthe purposes of this invention, carbon should be held to a maximum of.08%, by weight.

Application of the alloys Q and O of the present invention to the fieldof turbogenerator retaining rings will now be discussed. In order toappreciate the advantages of this alloy, the procedure followed with thealloys of Table I will first be considered. In the alloys of Table I,with carbides as the precipitate, the ingot is first solution heattreated to take the carbon into solution. After the solution heattreatment the ingot is mandrel forged in several steps, and then, in afinal working step, it is cold expanded over a tapered mandrel at arelatively low temperature for a relatively long time. This particularwor :ing step results in a very sharp increase of the hardness withincreasing deformation because of the simultaneous contributions of coldworking and precipitation. The sharp increase of the hardness and thusof the yield strength require high capacity presses to carry out theworking, and even though such presses are available, the reduction ofarea cannot proceed beyond a relatively low reduction because of therapid increase of the yield strength and the simultaneous decrease ofductility. It is obvious that Where only such low reductions areattainable, relatively great differences will result in the amount ofdeformation over the cross section of the Worked member, especially ifcertain areas precipitation harden before others. The non-uniformity ofworking and aging over the cross section will result in pronouncednonuniformity of the mechanical properties over the cross section of thering.

In order to show that the precipitation of chromium carbide or othercarbides takes place during cold working, the following experiment wasconducted: A rod of each of the alloys 1 and 2, about 0.5 inch indiameter was solution heat treated at 1050 C., quenched in water, rolledat 625 C. to 40%, 50% and 60% reduction, respectively, and then thespecimens were aged at 525 C. The measured diamond point hardness values(30 kg.) were:

Table V [Hardness (D.P.I-I.)]

As Rolled and As Rolled Aged, 25 Hrs. at Alloy As 525 C. I

Quenched Table VI Treatment: Hardness (D.P.H.) As quenched aftersolution heat treatment- 150 40% rolled at 630 C 230250 50% rolled at630 C 250-260, 60% rolled at 630 C 260-270 Aged from 24- hrs. to 300hrs. at 525 C.

to 550 C 350-370 It should be understood that even higher hardnessvalues can be attained than are indicated in the above table by longeraging or aging at higher temperatures, but the ductility would dropbelow acceptable values for the contemplated application if this were tobe done.

A comparison of Tables V and VI for the alloys shows strikingdiiferences which are important for this invention: (1) the hardnessesafter cold rolling at 630 C. are relatively low with the new alloys; (2)the reduction of area to reach a given hardness level is obviously muchhigher in the case of the new alloys than was the case with the oldalloys. The higher reduction of area possible with the new alloys,results in a much more uniformly worked material, and consequently, amember which will possess uniform mechanical properties throughout thebody thereof.

In the production of retaining rings, the properties of the new alloyssuggest the substitution of the economical tire rolling method whichuses relatively low capacity, inexpensive equipment, and requiresrelatively short times to roll the ring as compared to the high capacitypresses essential with present alloys. Since with these alloys theretaining rings can be formed while in a relatively soft condition thecost of processing is materially reduced. Thus, rough machining anddrilling can be performed while the retaining ring is in the unhardenedcondition,

and there is consequently less danger of crack formation at stressraising points. It should be noted that the uniform structuralconditions achieved in the working of these alloys carries over into theaging step in a manner such that the precipitation hardening occurs in avery uniform manner.

While alloys Q and O are particularly suitable for use as rings andwedges in generators, the other alloys of Table III are well suited forthis same purpose. However, due to the additional alloying elements inthese latter alloys, they are somewhat more expensive and, therefore,might better serve where their improved properties such as high yieldstrength and ductility are required. Some such uses are in underwaterdetection apparatus, mine sweeping devices, and in submarines.

v It will be understood by those skilled in the art that although thepresent invention has been described in connection with preferredembodiments, modifications and variations may be employed withoutdeparting from the underlying spirit and scope of the invention. It isintended to claim all such variations and modifications.

I claim as my invention:

1. An austenitic iron-base alloy consisting essentially of, by weight,from 16% to 30% manganese, from 2.25% to 3.25% titanium, carbon in anamount not exceeding 08%, and the balance iron except for incidentalimpurities and additives.

2. A nonmagnetic precipitation hardenable iron-base alloy composed of asits essential constituents, by weight, from about 25% to 26% manganese,from 1.8% to 3.5% titanium, a maximum of .08% carbon, and the balanceiron with small amounts of incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS1,574,782 Becker Mar. 2, 1926 2,048,167 Filling July 21, 1936 2,266,481Talbot Dec. 16, 1941 2,378,993 Franks June 26, 1945

1. AN AUSTENITIC IRON-BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT,FROM 16% TO 30% MANGANESE, FROM 2.25% TO 3.25% TITANIUM, CARBON IN ANAMOUNT NOT EXCEEDING 08%, AND THE BALANCE IRON EXCEPT FOR INCIDENTALIMPURITIES AND ADDITIVES.